In recent years, a high-voltage electrical storage device having high energy density has been desired as a power supply for driving an electronic device. A lithium-ion battery, a lithium-ion capacitor, and the like are considered to be a promising electrical storage device having such performance.
An electrode used for such an electrical storage device is normally produced by applying a composition (electrode slurry) that includes an active material and a polymer that functions as a binder to the surface of a collector, and drying the applied composition. The polymer used as a binder is required to have a capability to bind the active material, a capability to bind (bond) the active material and the collector, scratch resistance when winding the electrode, fall-off resistance (i.e., a fine powder of the active material or the like does not fall off from the layer (film) formed by applying and drying the composition (hereinafter may be referred to as “active material layer”) due to cutting or the like), and the like.
It was experimentally found that a capability to bind the active material, a capability to bind (bond) the active material and the collector, and the fall-off resistance have an almost proportional relationship. These properties may be comprehensively referred to herein as “binding capability”.
In recent years, use of a material having a high lithium occlusion capacity has been studied in order to increase the output and the energy density of the electrical storage device. For example, a method that utilizes a silicon material having a theoretical lithium occlusion capacity of up to about 4,200 mAh/g as the active material (see Patent Literature 1) is considered to be promising.
However, an active material that utilizes a material having high lithium occlusion capacity significantly changes in volume due to occlusion and release of lithium. Therefore, when a known electrode binder is applied to a material having a high lithium occlusion capacity, the active material may be removed due to deterioration in binding capability, and a significant decrease in capacity occurs due to charge and discharge, for example.
A technique that controls the amount of acid present on the surface of binder particles (see Patent Literature 1 and 2), a technique that utilizes an epoxy or hydroxyl group-containing binder (see Patent Literature 4 and 5), and the like have been proposed as a technique for improving the binding capability of an electrode binder. A technique that restrains the active material using the rigid molecular structure of a polyimide to suppress a change in volume of the active material has also been proposed (see Patent Literature 6).
A lithium-containing phosphoric acid compound having an olivine structure (olivine-type lithium-containing phosphoric acid compound) has attracted attention as a positive electrode active material that is highly safety. Since the olivine-type lithium-containing phosphoric acid compound has a structure in which phosphorus and oxygen are covalently bonded, the olivine-type lithium-containing phosphoric acid compound exhibits high thermal stability, and does not release oxygen even at a high temperature.
However, since the olivine-type lithium-containing phosphoric acid compound is characterized in the Li ion occlusion-release voltage is about 3.4 V, the output voltage is low. In order to compensate this drawback, attempts have been made to improve the properties of the electrode binder, the electrolyte solution, and the like (see Patent Literature 7 to 9).